Green’s Dictionary of Slang

(US) a leader, a boss.

1896		Ade Artie 103: If you can’t travel with the bell-cows, why stick to the gang?
1899		W.J. Kountz Billy Baxter’s Letters 41: When it comes to warm propositions he is certainly the bell cow.
1934		Z.N. Hurston Jonah’s Gourd Vine (1995) 93: John, dis is uh fine place tuh bring up our chillun. Dey won’t be seein’ no other kind uh folks actin’ top-superior over ’em and dat’ll give ’em spunk tuh be bell cows theyselves.
1966		in DARE.
1982		J. Ehrlichman Witness to Power 275: John Osborne was the bell-cow of the White House press corps.